Fabrication of sandwiched construction media

ABSTRACT

In one embodiment, holographic data storage medium includes a first thermoplastic substrate portion having a thickness less than approximately 2 millimeters and a second thermoplastic substrate portion having a thickness less than approximately 2 millimeters. A holographic recording material may be sandwiched between the first and second thermoplastic substrate portions. By making thermoplastic substrate portions sufficiently thin, edge wedge problems can be avoided.

This Application is a continuation of U.S. application Ser. No.10/781,439 filed Feb. 18, 2004, now U.S. Pat. No. 6,850,345 which is acontinuation of U.S. application Ser. No. 10/441,426 filed May 19, 2003,now abandoned, which is in turn a divisional of U.S. application Ser.No. 09/812,518, filed Mar. 20, 2001, now U.S. Pat. No. 6,611,365.

This invention was made with Government support under Agreement No.NMA02-97-9-1050 with the National Imagery and Mapping Agency of theUnited States Department of Defense. The Government has certain rightsin this invention.

FIELD

The present invention relates to holographic data storage media.

BACKGROUND

Many different types of data storage media have been developed to storeinformation. Traditional media, for instance, include magnetic media,optical media, and mechanical media to name a few. Increasing datastorage density is a paramount goal in the development of new orimproved types of data storage media.

In traditional media, individual bits are stored as distinct mechanical,optical, or magnetic changes on the surface of the media. For thisreason, medium surface area may pose physical limits on data densities.

Holographic data storage media can offer higher storage densities thantraditional media. In a holographic medium, data can be storedthroughout the volume of the medium rather than the medium surface.Moreover, data can be superimposed within the same media volume througha process called shift multiplexing. For these reasons, theoreticalholographic storage densities can approach tens of terabits per cubiccentimeter.

In holographic data storage media, entire pages of information can bestored as optical interference patterns within a photosensitive opticalmaterial. This can be done by intersecting two coherent laser beamswithin the optical material. The first laser beam, called the objectbeam, contains the information to be stored; and the second, called thereference beam, interferes with the object beam to create aninterference pattern that can be stored in the optical material as ahologram. When the stored hologram is later illuminated with only thereference beam, some of the reference beam light is diffracted by thehologram. Moreover, the diffracted light creates a reconstruction of theoriginal object beam. Thus, by illuminating a recorded hologram with thereference beam, the data encoded in the object beam can be recreated anddetected by a data detector such as a camera.

SUMMARY OF THE INVENTION

The invention is directed towards holographic data storage media,holographic data storage systems, and methods for making holographicdata storage media. The holographic data storage media may incorporatethermoplastic substrates having reduced substrate thicknesses. Moreover,in some embodiments, holographic data storage media incorporatethermoplastic substrates within a particular thickness range.

In one embodiment, a holographic data storage medium may include a firstthermoplastic substrate portion having a thickness less than orapproximately equal to 2 millimeters, a second thermoplastic substrateportion having a thickness less than or approximately equal to 2millimeters, and a holographic recording material sandwiched between thefirst and second thermoplastic substrate portions. By way of example,the first and second thermoplastic substrate portions may be made of atleast one of the following: polycarbonate, polymethylmethacrylate(PMMA), and amorphous polyolefin. The holographic recording material maybe made of a photopolymer.

The holographic data storage medium, for instance, may take the form ofa disk or a card. The first and second thermoplastic substrate portionsmay be injection molded substrate portions. As will be described indetail below, an edge wedge phenomenon associated with injection moldedthermoplastic substrates can make fabrication of holographic datastorage media challenging. To overcome problems introduced by the edgewedge phenomenon, the invention may involve the use of substrateportions with reduced thicknesses. For instance, each of the first andsecond thermoplastic substrate portions may have thicknesses less thanor equal to approximately 2 millimeters, less than or equal toapproximately 1.2 millimeters, or even less than or equal toapproximately 0.6 millimeters.

Optimal substrate thicknesses may have a lower limit determined by othervariables such as birefringence and stiffness. Therefore, in oneparticular embodiment, each of the first and second thermoplasticsubstrate portions have thicknesses less than 1.3 millimeters andgreater than 0.5 millimeters. 1.3 millimeters to 0.5 millimeters, forinstance, may define an optimal thermoplastic substrate thickness range.

In other embodiments, the invention may comprise a holographic datastorage system. The system may include a laser that produces at leastone laser beam and optical elements through which the laser beam passes.The system may also include a data encoder, such as a spatial lightmodulator, that encodes data in at least part of the laser beam. Inaddition, the system may include a holographic recording medium thatstores at least one hologram. The holographic recording medium, forinstance, may include one or more of the features mentioned above, suchas thin thermoplastic substrate portions. The system may also include adata detector, such as a camera, that detects the hologram.

In yet another embodiment, the invention may comprise a method offabricating holographic media. The method may include injection moldinga first substrate portion and a second substrate portion, and depositinga photopolymer between the first and second substrate portions.Injection molding the first and second thermoplastic substrate portions,for instance, may comprise injection molding the first and secondthermoplastic substrate portions to have sufficiently thin substratethicknesses. Depositing the photopolymer may comprise injecting thephotopolymer between the first and second substrate portions. Forinstance, for a disk shaped medium, the photopolymer may be injected bycenter dispensing the photopolymer through an inner diameter of thesubstrate portions of the medium. The method may also include forcingthe first substrate portion onto an upper reference plane and forcingthe second substrate portion onto a lower reference plane. Thephotopolymer may then be cured in situ.

Substrate thicknesses less than or equal to approximately 2.0millimeters, less than or equal to approximately 1.2 millimeters, orless than or equal to approximately 0.6 millimeters may be highlyadvantageous. In particular, substrate thicknesses in these ranges mayminimize the negative effects of the edge wedge phenomenon that isdescribed in detail below. Briefly, the edge wedge phenomenon is theresult of differential cooling of the thermoplastic material as itsolidifies in an injection molding cavity. The differential cooling, forinstance, can result in substrates that exhibit cusps at the substrateedges that are thicker than the average thickness of the substrate.

Other factors, such as birefringence and stiffness, however, may makethicker substrates more desirable. The range of 0.5 millimeters to 1.3millimeters, for instance, may define an optimal thermoplastic substratethickness range for sandwich construction holographic media.

Additional details of these and other embodiments are set forth in theaccompanying drawings and the description below. Other features, objectsand advantages will become apparent from the description and drawings,and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an optical arrangement for holographic recording.

FIG. 2 is an enlarged view of an exemplary 10 by 10 bit pixel array thatcan be stored on a holographic medium as a hologram.

FIG. 3 illustrates how sequential pages or pixel arrays may be stored ona holographic data storage medium.

FIGS. 4A and 4B illustrate an exemplary holographic data storage mediumin accordance with one embodiment of the invention.

FIG. 5 illustrates an exemplary system for fabricating holographic datastorage media.

FIGS. 6A and 6B illustrate an exemplary substrate exhibiting cusps.

FIG. 7 is an enlarged cross sectional view of an edge of a substratethat exhibits a cusp caused by the edge wedge phenomenon.

FIG. 8 illustrates a system suitable for reading and writing to aholographic recording medium.

DETAILED DESCRIPTION

FIG. 1 illustrates an optical arrangement for holographic recording. Asshown in FIG. 1, laser 10 produces laser light that is divided into twocomponents by beam splitter 14. These two components generally have anapproximately equal intensity and may be spatially filtered to eliminateoptical wave front errors.

The first component exits beam splitter 14 and follows an object path.This “object beam” may then pass through a collection of object beamoptical elements 18A–18E and a data encoder such as a Spatial LightModulator (SLM) 20. For instance, lens 18A may expand the laser lightand lens 18B may condition the laser light so that the photons aretraveling substantially parallel when they enter SLM 20.

SLM 20 may encode data in the object beam, for instance, in the form ofa holographic bit map (or pixel array). FIG. 2, for instance, shows anenlarged view of an exemplary 10 by 10 bit pixel array. The encodedobject beam may pass through lenses 18C, 18D, and 18E beforeilluminating a holographic recording media plane 21. In thisconventional “4F” configuration, lens 18C is located one focal lengthfrom SLM 20 and one focal length from Fourier transform plane 24A. Lens18D is located one focal length from Fourier transform plane 24A and onefocal length from image plane 22A. Lens 18E is located one focal lengthfrom image plane 22A and one focal length from Fourier transform plane24B.

The second component exits the beam splitter 14 and follows a referencepath. This “reference beam” may be directed by reference beam opticalelements such as lenses 26 and mirrors 28. The reference beamilluminates the media plane 21, interfering with the object beam tocreate a hologram on medium 25. By way of example, medium 25 may takethe form of a disk shaped medium or a card shaped medium. For instance,if the medium takes the form of a disk, the disk may be rotatable withina holographic disk drive to read and write data.

In order to provide increased storage density, storage medium 25 istypically located in proximity to one of the Fourier transform planes.Using an optical arrangement similar to that shown in FIG. 1, forexample, the data encoded in the object beam by SLM 20 can be recordedin medium 25 by simultaneously illuminating the object and referencepaths.

After a hologram has been stored on the medium 25, the data encoded inthe hologram may be read, again, e.g., using an optical arrangementsimilar to that shown in FIG. 1. For readout of the data, only thereference beam is allowed to illuminate the hologram on medium 25. Lightdiffracts off the hologram stored on medium 25 to reconstruct or“re-create,” the object beam, or a beam of light that is substantiallyequivalent to the original encoded object beam. This recreated objectbeam passes through lens 18F, permitting a reconstruction of the bit mapthat was encoded in the object beam to be observed at image plane 22B.Therefore, a data detector, such as camera 30 can be positioned at imageplane 22B to read the data encoded in the hologram.

The holographic bit map encoded by SLM 20 comprises one “page” ofholographic data. For instance, the page may be an array of binaryinformation that is stored in a particular location on the holographicmedium as a hologram. By way of example, a typical page of holographicdata may be 1000 bit by 1000 bit pixel array that is stored in 1 squaremillimeter of medium surface area, although the scope of the inventionis not limited in that respect. Because holographic data is storedthroughout the medium volume, however, sequential pages may beoverlapped in the recording process by a process called shiftmultiplexing.

In one type of shift multiplexing of holographic data pages, sequentialpages are recorded at shifted locations around the medium. The shiftdistances are typically much less than the recorded area in onedimension (the down-track dimension) and approximately equal to therecorded area in the other dimension (the cross-track dimension). FIG.3, for example, illustrates how sequential pages may be stored on medium32. A portion 33 of medium 32 is enlarged for illustrative purposes. Asshown, sequential pages of data 34 are overlapped in the down-trackdimension 36. Later pages 38 in the sequence of pages also overlap oneanother in the down-track dimension 36 but do not overlap pages in thecross-track dimension 39. The respective pages of data, for instance,may each cover approximately 1 square millimeter of surface area on themedium. The down-track dimension, for instance may be approximately 10microns, while the cross-track dimension may be approximately 1millimeter.

In an alternative type of shift multiplexing, sometimes referred to asphase correlation multiplexing, sequential pages are overlapped in therecording process in both the cross track dimension and the down trackdimension.

In order to precisely locate track locations, a holographic data storagesystem may implement one or more tracking techniques. Exemplary trackingtechniques, for instance, are described in commonly assigned U.S. Pat.No. 6,625,100. The content of the above-identified application is herebyincorporated herein by reference in its entirety.

FIGS. 4A and 4B illustrate an exemplary embodiment of a holographic datastorage medium 40. FIG. 4A is a top view illustrating the disk shape ofmedium 40. FIG. 4B is a cross sectional view illustrating the sandwichconstruction of medium 40. For instance, medium 40 includes a substratehaving a first thermoplastic substrate portion 41 and a secondthermoplastic substrate portion 42. Holographic recording material 43may comprise a photopolymer that is sandwiched between the respectivesubstrate portions 41, 42. By way of example, the substrate portions 41,42 may be made of at least one of the following: polycarbonate,polymethylmethacrylate, or amorphous polyolefin. One of these exemplarythermoplastics or another thermoplastic, for example, may be used tomake substrate portions 41, 42 in an injection-molding tool.

FIG. 5 illustrates an exemplary system for fabricating high opticalquality holographic data storage media. The system 50 may include atleast two flat portions 51, 52. To fabricate a holographic medium 40,thermoplastic substrate portions 41, 42 may be forced against therespective flat portions 51, 52 so that the outer surface of thesubstrate portions 41, 42 substantially conform to respective flatportions 51, 52. For example, a force may be applied via a vacuum, orthe force may come from hydraulic pressure, e.g., if holographicrecording material 43 is injected between the substrate portions.

A holographic recording material 43 may be dispensed between therespective thermoplastic substrate portions 41, 42. For instance, for adisk shaped medium, the holographic recording material 43 may beinjected by center dispensing a photopolymer through an inner diameterof the thermoplastic substrate portions 41, 42. In center dispensing,the photopolymer material flows radially to fill the cavity between thesubstrate portions 41, 42. Photopolymer material flow lines may besymmetric to the disk shaped medium. As mentioned above, if theholographic recording material is dispensed via injection, the injectionprocess may also force the thermoplastic substrate portions 41, 42against the respective flat portions 51, 52.

By way of example, the first and second flat portions 51, 52 may bepositioned to define upper and lower reference planes. After dispensingthe holographic recording material, e.g., via the center dispensingmethod mentioned above, the upper flat portion 51 or lower flat portion52 may be adjusted slightly so that the parallelism of medium surface ismaintained throughout medium fabrication. For high optical qualityholographic media, substrate portions 41, 42 may need to be parallel towithin one optical fringe.

For example, an interferometer may be implemented to measureparallelism. Alternatively, the system 50 may be mechanicallypre-calibrated to ensure that the upper and lower surfaces of theresulting fabricated media will be sufficiently parallel. If the systemis mechanically pre-calibrated to ensure parallelism, the positions ofthe upper and lower flat portions 51, 52 may be carefully pre-defined sothat the upper and lower surfaces of the resulting fabricated media willbe sufficiently parallel. In addition, if the system is mechanicallypre-calibrated, an interferometer used to monitor parallelism may beunnecessary.

Holographic recording material, for instance, may be a photopolymermaterial. After the medium has been fabricated, the holographicrecording material may have a thickness of approximately one millimeter.Thermoplastic substrate portions 42, 43 may each have thickness lessthan or equal to approximately 2 millimeters, less than or equal toapproximately 1.2 millimeters, or even less than or equal toapproximately 0.6 millimeters. Thus, the total thickness of theholographic recording medium may be less than or equal to approximately5 millimeters, less than or equal to approximately 3.4 millimeters, oreven less than or equal to approximately 2.2 millimeters. As describedbelow, however, an optimal range of respective substrate thicknesses mayfall within 0.5 millimeters to 1.3 millimeters.

Thermoplastic substrate portions 42, 43, for instance, may be comprisedof a suitable thermoplastic such as polycarbonate,polymethylmethacrylate, amorphous polyolefin, or the like. Moreover,thermoplastic substrate portions 42, 43 may be fabricated by aninjection molding process. Injection molding the substrates, forinstance, can provide many advantages in fabrication.

For instance, injection molded substrates can be mass-produced atrelatively low cost. Moreover, the injection molds may be adapted toprovide the substrates with additional features such as surfacevariations that are optically detectable. These optically detectablesurface variations can carry precision tracking and/or pre-formatinformation. Commonly-assigned U.S. Pat. No. 6,625,100, for instance,describes how prerecorded surface variations may provide precisiontracking features to a holographic medium. For these and other reasons,it may be highly advantageous to fabricate the respective substrateportions by injection molding.

Injection molding the thermoplastic substrate portions, however, raisesadditional challenges. For instance, injection molding can result insubstrate portions exhibiting an “edge wedge” phenomenon. The “edgewedge” phenomenon is the result of the cooling and solidification of thethermoplastic in an injection mold. As a thermoplastic material coolsand solidifies in an injection mold, the thermoplastic material at theinner and outer edges of the mold may cool at a different rate than therest of the thermoplastic material.

This differential cooling can result in a substrate thickness that isnon-uniform. For instance, small cusps at the edges of an injectionmolded disk substrate may have a thickness that is greater than theaverage disk thickness. The presence of such cusps is referred to as the“edge wedge” phenomenon or “edge wedge” problem.

FIGS. 6A–6B illustrate an exemplary substrate 60 exhibiting a cusp 62caused by the edge wedge phenomenon. FIG. 6A is a top view of substrate60 and FIG. 6B is an enlarged cross sectional view of the outer edges ofsubstrate 60. As shown in FIG. 6B, outer edges of substrate 60 exhibitsa cusp 62 that makes the outermost substrate thickness larger than theaverage substrate thickness. In addition, at any location within themold cavity wherein there is a thermal gradient in the molding tool, anedge wedge phenomenon may occur. For example, one or more edge wedgecusps may also be observed at or near the inner diameter of substrate60.

FIG. 7 is an enlarged cross sectional view of an edge of substrate 70exhibiting a cusp 72 caused by the edge wedge phenomenon. For example,FIG. 7 may correspond to either the inner or outer edge of a disk shapedsubstrate. As shown, the extent of the edge wedge phenomenon can bemeasured with variables X and Y. The variable Y can be used to measurethe difference between the maximum thickness of the disk at a cusp andthe average thickness of the disk. The variable X can be used to measurethe distance between an edge of the disk and the point on the disk wherethe disk thickness becomes substantially uniform. By way of example, thevariable X may be typically about 1–2 millimeters and the variable Y maybe on the order of tens of microns.

Experimental data has shown that the edge wedge phenomenon increases assubstrate thickness increases. For instance, as the thickness of aninjection molded substrate increases, the variables X and Y, as shown inFIG. 7, increase. In other words, the size of the cusps that resultbecause of the edge wedge phenomenon become larger as the thickness ofthe substrate increases. And similarly, the edge wedge phenomenondecreases as substrate thickness decreases.

The edge wedge phenomenon presents a significant obstacle to thefabrication of high optical quality holographic media. Referring againto FIG. 5, substrate portions 41 and 42 are forced against therespective flat portions 51, 52. However, if substrate portions 41, 42exhibit cusps, e.g., as the result of the edge wedge phenomenon, thepresence of cusps on the edges of substrate portions 41, 42 mayundermine the ability to achieve uniform holographic recording materialthickness. If substrate portions 41, 42 exhibit cusps, for example, itcan be particularly difficult to ensure that the outer surface ofsubstrate portions 41, 42 are parallel to within one optical fringe.

In short, the presence of cusps can undermine the ability to create ahigh optical quality holographic data storage medium. For instance, ifsubstrate portions cannot be uniformly forced against reference planes(such as flat portions 51, 52 as shown in FIG. 5), high optical qualityholographic data storage disks fabrication techniques may beineffective. Moreover, even if disk fabrication is still possible, theusefulness of the disk can be reduced by the presence of cusps on thesubstrate portions. For instance, the storage density of the disk may bereduced if, during media fabrication, the substrate portions cannot bemade to be sufficiently parallel to cause the holographic recordingmaterial to have a substantially uniform thickness.

To overcome problems relating to the edge wedge phenomenon, substrateportions may be fabricated to be sufficiently thin. In this manner, theedge wedge phenomenon may be reduced dramatically. In particular,thinner injection molded substrates exhibit smaller edge wedgephenomenon cusps. For instance, referring again to FIG. 7, the variableY can be reduced significantly when the thickness of injection moldedsubstrates is reduced. Specifically, substrate portion thickness lessthan or equal to approximately 2 millimeters, less than or equal toapproximately 1.2 millimeters, and even less than or equal toapproximately 0.6 millimeters are useful to overcome edge wedgeproblems.

As mentioned above, however, other media design variables such asbirefringence and media stiffness may pose limitations on how thin auseful substrate can be made. In general, thicker substrates are stifferand have lower birefringence. Thus, as described in the example below,there may exist an optimal substrate thickness range in which a numberof variable fall within acceptable range for a high quality sandwichconstruction holographic data storage medium.

EXAMPLE

In experiments, thermoplastic materials such as polycarbonate,polymethylmethacrylate, and amorphous polyolefin are injection molded tofabricate 120-millimeter diameter substrates and 130-millimeter diametersubstrates having thicknesses of 2.0 millimeters, 1.2 millimeters and0.6 millimeters. The edge wedge is then measured for each respectivesubstrate thickness, e.g., by measuring the variable Y as shown in FIG.7. Holographic data storage media are then fabricated using therespective substrates of differing thicknesses and the media fabricationtechniques described above.

The experimental results show that the 2.0 millimeter substratestypically exhibit an edge wedge of approximately Y=20 microns, the 1.2millimeter substrates typically exhibit an edge wedge of approximatelyY=10 microns, and the 0.6 millimeter substrates typically exhibit anedge wedge of approximately Y=4 microns. Thus, the results show that theedge wedge phenomenon decreases with decreasing substrate thicknesses.

Also measured are the birefringence of each substrate (i.e., a measureof the variation of the index of refraction with orientation in thematerial). The experimental results show that birefringence isapproximately 30 nanometers for the 2.0 and 1.2 millimeter substratesand increases to approximately 60 nanometers for the 0.6 millimetersubstrates. Thus, the results show that birefringence increases withdecreasing substrate thickness, and in particular, that birefringenceincreases dramatically for substrate thickness less than orapproximately equal to 0.6 millimeters.

The experimental results described above clearly show the edge wedgeadvantages that can be achieved by decreasing the size of the substrateportions in a sandwiched construction holographic data storage medium.Substrate portions having a thickness of 2.0 millimeters or less appearto be adequate for some implementations, but substrate portions having athickness of 1.2 millimeters or less exhibit smaller cusps. Moreover,substrate portions having a thickness of 0.6 millimeters or less hadeven smaller cusps. Substrate portions having a thickness of 0.6millimeters had an acceptable, yet much higher birefringence than thethicker substrate portions. As substrate portions get much thinner than0.5 millimeters, for example, birefringence may increase to unacceptablelevels.

In addition to overcoming the edge wedge problems, thinner substratesalso produce advantages in overall media thickness. Thinner substrates,for instance, can reduce the total material cost of an individualmedium. There may, however, be a lower limit to the thickness of asubstrate portion that could be dictated, e.g., by substratebirefringence, stiffness, and/or other factors such as environmentalprotection capability. In short, the optimal thickness range forthermoplastic substrate portions for use in a sandwich constructionholographic data storage medium is approximately 0.5 millimeters to 1.3millimeters.

As mentioned above, substrate portions may be formed from athermoplastic such as a polycarbonate, polymethylmethacrylate, amorphouspolyolefin, or the like. For injection molded substrates, severalfactors may determine which substrate material is best suited for mediafabrication. For instance, the substrate material should be capable ofconfining a viscous photopolymer during the fabrication process. Inaddition, the material should be capable of exhibiting surfacevariations, e.g., embossed or molded surface variations, that can carryprecision tracking and/or pre-format information. The material should becapable of encapsulating the holographic recording material to protectthe holographic recording material from environmental contamination. Inaddition, as discussed above, the material should have relatively lowbirefringence. Birefringence is generally a measure of the variation ofthe index of refraction with orientation in the material. Largevariations in the index of refraction with orientation, for instance,are generally undesirable for holographic media substrate materials.

Additionally, a material should be chosen so that surface reflection andintrinsic optical scattering are minimized as much as possible.Moreover, the medium may need an anti-reflection coating. Finally,material cost may be a factor. Considering all these factors, thethermoplastic material referred to as amorphous polyolefin (APO) appearsto be well suited as a substrate material for use in holographic media.In addition to meeting the design criteria above, APO does not absorbwater vapor. Therefore, if APO is used as a holographic mediumsubstrate, an anti-reflective coating can be applied to the mediumwithout the need to bake or de-gas the substrate.

As described above, the optimal thickness range falls betweenapproximately 0.5 millimeters and 1.3 millimeters. Birefringence may betoo high if the substrate thickness is less than 0.5 millimeters, andedge wedge may become a problem if substrate thickness is greater than1.3 millimeters. Therefore 0.5 millimeters to 1.3 millimeters representsthe optimal range for thermoplastic substrates used in sandwichconstruction holographic data storage media, and APO appears to be themost suitable thermoplastic material for such substrates.

FIG. 8 illustrates a system 100 suitable for reading and writing to aholographic recording medium. System 100 includes at least one laser 102that produces laser light 104. Laser light 104 passes through opticalelements 106. For instance, optical elements 106 may include one or morebeam splitters, lenses and mirrors. A data encoder, such as SLM 108 maybe positioned within the optical elements to encode data in the laserlight 104. By way of example, the optical elements 106 may conform tothe optical arrangement shown in FIG. 1, although the scope of theinvention is not limited in that respect. Medium 110 is positioned whereit can be written with holographic bit maps. Medium 110, for instancemay include one or more of the features described above, including thinsubstrate portions. Each substrate portion, for instance, may have athickness less than or equal to approximately 2 millimeters, less thanor equal to approximately 1.2 millimeters, or even less than or equal toapproximately 0.6 millimeters. Moreover, each substrate portion may fallwithin the optimal thickness range of approximately 0.5 millimeters to1.3 millimeters.

Data detector 112 such as a camera is positioned to detect data encodedbit maps on medium 110. A tracking detector (not shown) such as a PSD, asegmented detector array, a two-element photodetector or the like, maybe positioned to detect light diffracted from medium 110 in a mannerthat enables system 100 to accurately locate track locations on medium110.

In the system, at least one laser 102 may be carried on a record/readhead (not shown). Additional lasers (not shown) may also be carried onthe record/read head. In this manner, laser 102 may be properlypositioned to read and write holograms on the medium 110.

Various embodiments of the invention have been described. For example, asandwich construction holographic data storage medium has beendescribed. The substrate portions of medium may be of adequate thicknessto overcome edge wedge problems. Moreover, the medium may form part of aholographic data storage system. These and other embodiments are withinthe scope of the following claims.

1. A method of fabricating a holographic data storage medium comprising:injection molding a first substrate portion and a second substrateportion; depositing a photopolymer between the first and secondsubstrate portions; forcing the first substrate portion onto an upperreference plane of a holographic data storage medium fabrication system;forcing the second substrate portion onto a lower reference plane of theholographic data storage medium fabrication system; and curing thephotopolymer.
 2. The method of claim 1, wherein injection molding thefirst substrate portion and the second substrate portion compriseinjection molding the first and second substrate portions to have firstand second substrate thicknesses less than approximately 2.0 millimetersand greater than approximately 0.5 millimeters.
 3. The method of claim 1wherein depositing a photopolymer between the first and second substrateportions comprises injecting the photopolymer between the first andsecond substrates by center dispensing the photopolymer through an innerdiameter of at least one of the substrate portions, and wherein centerdispensing the photopolymer forces the first and second substrate ontothe upper and lower reference planes respectively.
 4. The method ofclaim 1, further comprising mechanically pre-calibrating the upper andlower reference planes of the holographic data storage mediumfabrication system.
 5. The method of claim 4, further comprisingmechanically pre-calibrating the upper and lower reference planes of theholographic data storage medium fabrication system to ensure that upperand lower surfaces of a resulting fabricated medium are substantiallyparallel.
 6. The method of claim 1, wherein injection molding thesubstrate portions includes embossing one or both of the substrateportions to include tracking or pre-format information.
 7. The method ofclaim 1, further comprising applying an anti-reflective coating on oneor both of the substrate portions.
 8. The method of claim 7, wherein thesubstrate portions comprise amorphous polyolefin and wherein applying toanti-reflective coating does not include baking or de-gassing of thesubstrate portions.
 9. A method of creating a data storage mediumcomprising: mechanically pre-calibrating an upper reference plane and alower reference plane of a holographic data storage medium fabricationsystem; forcing the first substrate portion onto the upper referenceplane of the holographic data storage medium fabrication system; forcingthe second substrate portion onto a lower reference plane of theholographic data storage medium fabrication system; depositing a polymerbetween first and second substrate portions; and curing the polymer. 10.The method of claim 9, wherein the medium comprises a holographic mediumand the polymer comprises a photopolymer.
 11. The method of claim 9,wherein depositing the polymer between the first and second substrateportions comprises injecting the polymer between the first and secondsubstrates by center dispensing the polymer through an inner diameter ofat least one of the substrate portions and wherein center dispensing thepolymer forces the first and second substrate portions onto the unperand lower reference planes respectively.
 12. The method of claim 9,wherein mechanically pre-calibrating the upper and lower referenceplanes of the halographic data storage medium fabrication system ensuresthat upper and lower surfaces of the medium are substantially parallel.13. The method of claim 9, further comprising embossing one or both ofthe substrate portions to include tracking or pre-format information.14. The method of claim 9, further comprising applying ananti-reflective coating on one or both of the substrate portions. 15.The method of claim 14, wherein the substrate portions compriseamorphous polyolefin and wherein applying the anti-reflective coatingdoes flat include baking or de-gassing of the substrate portions.
 16. Amethod of creating a sandwiched construction holographic mediumcomprising: injection molding a first substrate portion and a secondsubstrate portion; applying an anti-reflective coating on one or both ofthe substrate portions; depositing a photopolymer between the first andsecond substrate portions to force the first and second substrateportions onto an upper and lower reference planes respectively; andcuring the photopolymer.
 17. The method of claim 16, wherein depositinga photopolymer between the first and second substrate portions comprisesinjecting the photopolymer between the first and second substrates bycenter dispensing the photopolymer through an inner diameter of at leastone of the substrate portions.
 18. The method of claim 16, furthercomprising embossing one or both of the substrate portions to includetracking or pre-format information.
 19. The method of claim 16, whereinthe substrate portions comprise amorphous polyolefin end whereinapplying the anti-reflective coating does not include baking orde-gassing of the substrate portions.
 20. A method of fabricating amedium comprising: injection molding a first substrate portion and asecond substrate portion; forcing the first substrate portion onto anupper reference plane of a holagraphic data storage medium fabricationsystems; forcing the second substrate portion onto a lower referenceplane of the holographic data storage medium fabrication system;depositing a polymer between the first and second substrate portions;and curing the polymer.
 21. The method of claim 20, wherein the mediumcomprises a sandwiched construction holographic medium and the polymercomprises a photopolymer.
 22. The method of claim 20, wherein injectionmolding the first substrate portion and the second substrate portioncomprise injection molding the first and second substrate portions tohave first and second substrate thicknesses less than approximately 2.0millimeters and greater than approximately 0.5 millimeters.